The phone number and mobile number fields are designed to handle both US and Non-US phone numbers. If you would like to restrict it to be US only, please add the following code to your layouts (or a javascript file in your layouts).
$("[data-validate*='tel']").each(function () {
var inputElement = document.getElementById($(this)[0].id);
inputElement.addEventListener('keydown', enforceFormat);
inputElement.addEventListener('keyup', formatToPhone);
});